1. Field of the Invention
The invention relates to the use of salts of polyaspartic acids as agents to assist in the milling (comminution and related processing) of mineral materials in aqueous suspension. The invention also relates to a milling process employing salts of polyaspartic acids as auxiliary milling agents. Further, the invention relates to aqueous suspensions of finely divided minerals which suspensions contain the described agent(s) and are intended for pigment applications in the paper industry.
2. Description of the Related Art
Those skilled in the art, in order to use such an aqueous suspension of minerals in the field of pigments, will seek to carry out comminution of the minerals such that an aqueous suspension is produced in which the particle sizes are extremely fine, indeed as fine as possible, in particular less than a few microns.
At the same time, it is desirable that such a suspension have a Brookfield viscosity which is sufficiently low that any risk of sedimentation, setting, or agglomeration is eliminated. In this way the suspension can be manipulated easily by the user without agitation, even after being allowed to stand in a tank for several days. Such a suspension should also have the maximum possible content of the mineral materials, so as to minimize the manufacturing-related and transport-related costs which are correlated to the amount of water present.
It is problematic to those skilled in the art to devise milling operations in aqueous media, i.e. operations wherein the sizes of mineral particles are reduced and their specific surface is increased, which operations will result in the described ideal suspensions which combine all the desired fundamental qualities.
The problems which arise during the milling operations in aqueous media at high concentrations of the mineral materials, said problems including notably a rapid increase in viscosity which is experienced, are due to the fineness of the particles. Setting of the suspensions in the storage tanks can occur, or clogging of the milling implements. Such problems are not encountered in operations confined to dispersion of a mineral material in a liquid where the object is to effect the dispersion without changing the sizes of the particles of the mineral material.
It has been long known to use water-soluble polymers based on polyacrylic acid or its derivatives as milling agents (Eur. Pats. 0,100,947, 0,542,643, and 0,542,644), for the purpose of preparing aqueous suspensions of minerals which satisfy the abovementioned quality criteria. However, a drawback of such known milling agents is poor biodegradability.
The prior art did not offer those skilled in the art a satisfactory means of solving this problem of using a milling agent for mineral materials which enables the preparation of aqueous suspensions of finely divided mineral materials of high concentration in such materials, wherewith the minerals do not settle and the suspensions are pumpable and are environmentally benign.